1. Field of the Invention
The present invention relates to a method and apparatus for transmitting signals on a transmission line and, more specifically, to a method and apparatus for transmitting signals on a transmission line in an instrinsically safe explosion proof transmission system (hereinafter referred to as an "ISEP transmission system").
In particular, the present invention concerns the communication of signals in a communication standard "Field Bus." Standardization of such a field bus is currently under way in organizations such as the IEC (International Electrotechnical Commission). The present invention is applicable, for example, in industrial instrumentation and control applications, wherein a DC power is supplied to terminals connected to a transmission line and communication is effected between the terminals by superimposing an AC signal on the DC signal.
2. Description of the Related Art
FIG. 7 illustrates an example of a general configuration of an ISEP transmission system. As shown in FIG. 7, the ISEP transmission system comprises a bus power source 1, inductors 2a and 2ba Zener barrier 3, terminators 4a and 4b, a transmission line 5, a master terminal 6, and terminals 7. It should be understood that D denotes a dangerous area in which, for example, an explosive gas is present or is likely to be present, while S denotes a safe area which is free of such a gas.
The transmission line 5 comprises, for example, a pair of conductive wires. The Zener barrier 3, in its normal state, can be regarded simply as a series resistor and, as shown in FIG. 7, the Zener barrier 3 is provided at a boundary of the safe area S and the dangerous area D in order to limit the voltage and current flowing through the dangerous area D.
The bus power source 1 comprises, for example, a fixed voltage source for supplying a DC power to the terminals 7 via the Zener barrier 3 and the transmission line 5. As shown in FIG. 7, the terminals 7 are connected in a multidrop fashion to the transmission line 5. Likewise, the master terminal 6 is also connected to the transmission line 5 via the barrier 3 such that the-master terminal 6 and the terminals 7 can communicate. It should be understood that, typically, only one of the terminals 7 communicates with the master terminal 6 at any given time. It should also be understood that, typically, each of the terminals 7 constantly draws a portion of the DC power, as will be described later.
The inductors 2a and 2b are provided between the bus power source 1 and the master terminal 6 to separate the DC power from the AC signals which are transmitted between the master terminal 6 and terminals 7. Further, the terminators 4a and 4b are connected to opposite ends of the transmission line 5, and each comprises, for example, a resistor and a capacitor for eliminating DC component in the transmitted signal.
FIGS. 8(a) and 8(b) are graphs illustrating a conventional relationship between the current drawn into one of the terminals 7 of the ISEP transmission system of FIG. 7 and the corresponding voltage along the transmission line 5 of the system during transmission and non-transmission states. This relationship is described in, for example, Section 7.3 (page 6), with reference to FIG. 5, of IFC Document No. 910425 28, "Guidelines for Physical Layer Testing Low Speed, Voltage Mode," which was distributed during a meeting of the IFC Physical Layer Technical Working Group on Apr. 9-11, 1991 in the Hotel Softel, Bloomington, Minn., U.S.A.
As shown in FIG. 8(a), during non-transmission, the terminal draws a constant current of 8 mA, a portion of which is used by the terminal as a power source for its internal circuitry. During transmission, the terminal sends a signal by changing the amount of current it draws from the transmission line 5 from a constant 8 mA to an AC current of 15 mA peak-to-peak (hereinafter referred to as "pp"). As shown in FIG. 8(b), the 15 mA pp current corresponds to a voltage signal on the transmission line 5 having a magnitude of 0.75 V pp, by virtue of the 50 .OMEGA. impedance of the transmission line 5.
If, for example, the 15 mA pp current is represented by the Manchester code, the duty ratio between the high level and the low level of the signal is equal, and the average value of the current drawn by the terminal during transmission is 8 mA which, incidentally, is the same as during non-transmission. This average value of 8 mA is one in which a slight margin is added to the average value of 15 mA pp, and is greater than the value which the terminal requires as its internal power source. In addition, the reason for changing the current in obtaining a voltage signal is that this method excels in that, as compared with the case where the voltage is directly changed, the signal voltage is constant irrespective of the presence or absence of the Zener barrier 3, and that protection of the transmission line 5 from short-circuiting is facilitated.
According to the above-described conventional transmission method, during non-transmission, each of the terminals 7 continuously draws from the transmission line 5 a current equal to the average current drawn during transmission. It should be noted that the current drawn during non-transmission, however, exceeds that which is required by the terminal as its own power source. Further, because the total amount of current supplied by the bus power source 1 to each of the terminals 7 is restricted to a certain value by the Zener barrier 3, the number of terminals which we may connect to the transmission line 5 is unnecessarily limited. For example, assuming that the maximum amount of current which can flow through the dangerous area D via the Zener barrier 3 is 36 mA, and assuming that each terminal draws an average current of 8 mA during both transmission and non-transmission, it follows that, at most, only 4 terminals can be connected to the transmission line 5 at any given time.